There are five minor fast days in Judaism and two major fasts: Yom Kippur and Tisha B’Av. The only fast I observe is Yom Kippur. This is the Day of Atonement, the most solemn day on the Jewish calendar. However, Yom Kippur, though solemn, is not a sad day, as is Tisha B’Av. It is not a day of mourning. It is our opportunity to make amends for our sins toward G-d and toward others (which must be made directly toward those whom we have wronged). So, in a sense, this is a happy occasion.
The rules of fasting for Yom Kippur are quite strict, unlike the minor fasts which have many leniencies. The fast lasts more than 24 hours (about 26 hours) and it is a COMPLETE fast: no food and no water; no brushing of teeth; no touching of food to one’s lips. Still, in certain cases for health reasons, such as pregnant women, fasting is not only waived but it is NOT permitted. Also, young children need not fast. The fasting must be part of the purifying rituals (prayers, avoidance of wearing shoes made of leather, wearing white, etc.) of the day and cannot merely be a substitute for them. Work is forbidden just as on the Sabbath. Yet for all the solemnity attached to Yom Kippur, if a matter of saving a human life should arise, the latter takes precedence over the observance of this holy day. Further, all vows made on Yom Kippur for the upcoming year are abrogated in advance. Although we try to behave like the angels on this day, we realize our imperfections as well.